TY  - JOUR
A1  - Roland, Steffen
A1  - Kniepert, Juliane
A1  - Love, John A.
A1  - Negi, Vikas
A1  - Liu, Feilong
A1  - Bobbert, Peter
A1  - Melianas, Armantas
A1  - Kemerink, Martijn
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells
JF  - The journal of physical chemistry letters
N2  - We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.
Y1  - 2019
U6  - https://doi.org/10.1021/acs.jpclett.9b00516
SN  - 1948-7185
VL  - 10
IS  - 6
SP  - 1374
EP  - 1381
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Dispersive and steady-state recombination in organic disordered semiconductors
JF  - Physical review : B, Condensed matter and materials physics
N2  - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevB.96.245204
SN  - 2469-9950
SN  - 2469-9969
VL  - 96
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Hofacker, Andreas
A1  - Neher, Dieter
T1  - Dispersive and steady-state recombination in organic disordered semiconductors
JF  - Physical review : B, Condensed matter and materials physics
N2  - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevB.96.245204
SN  - 2469-9950
SN  - 2469-9969
VL  - 96
SP  - 5640
EP  - 5649
PB  - American Physical Society
CY  - College Park
ER  -